texts: How to retrieve from NASA Horizons site the needed data to plot analemmas for other planets?

texts:
I posted such a question in space.stackexchange ( https://space.stackexchange.com/questions/46077/which-are-the-correct-input-parameters-for-nasa-horizons-query-to-get-the-right?noredirect=1#comment150190_46077 ), but probably it fits better here, being it a "topocentric problem", although for other planets than Earth; we could call it "eso-astronomy" ;-)
I am trying to figure out how to retrieve the right data from Nasa Horizons to plot sun analemmas as seen from other worlds; I found a query which works fine for Earth, but upon changing the observer location from Earth to other planets, things get weird, and I get wrong curves w.r.t "official" curves (although there is not a single analemma curve for each planet, but different curves for different times).
This is the query I am using for Earth analemma centered on Greenwich at 12.00:
Result:

Using same coordinates on mars, a longer period, and 1477 minutes as interval (martian day length), I don't get the right plot (link):

The right analemma on Mars is this:

I noticed that Earth analemmas are plotted against Azimuth, but Mars analemmas are plotted against Equation of Time. Why? And how can I do by myself such a plot in Excel?


texts:
The below photos were taken yesterday (August 4, 2018) at about 10 p.m. from the plane over the Black Sea.

We were seeing the light dot on the left for at least 10-20 minutes. It looked like a bright star, but there were no other stars.

How likely that it was Mars?


texts:
I want to see Mars polar caps by myself. Is it feasible to make a picture of mars like this at home or to achive this very expensive telescope is required?

P.S. Maybe someone can give me a link, where it is shown how polar caps are changing during the seasons? Many pictures or animation.


texts: What are practical considerations for backyard radio-astronomy detection of black holes?

texts:
Evidently, direct observation of a black hole for an amateur astronomer, such as described for what professionals do in the question "How are black holes found?" would be nigh on impossible, so the observations would be based on their effects of the surrounding matter (specifically, accretion disks and jets) and more likely detecting the radiation signals.

Given that the locations of many black holes are known, what practical considerations would a backyard radio-astronomer need to consider to attempt to do detect black holes?

Amateur radio-astronomy detection of pulsars and gamma ray bursts are discussed practically in Introduction to Radio Astronomy (Society of Radio Astronomers), this question is to see what practical considerations would be needed to extend those techniques to detecting black holes.


texts: How accurately could late 50s - early 60s humans have mapped the solar system?

texts:
What if early SETI investigations had focused on our own solar system, rather than distant stars?
Given the technology that existed around the 1950s, how much surveillance would humanity be able to do? This is in the era between Sputnik and Apollo 1, so various governments and scientific organizations would have had different equipment and personnel to devote to the task. So this change of focus would have impacted the way various governments, scientific organizations and civilian groups approached the search for intelligent life in space, and altered the focus of technological development.
What were the near-space observation capabilities of radio telescopes and related devices at that time? For example if Cornell University's Project Ozma or Ohio State University's Big Ear were used to look more closely at our own solar system, rather than distant stars, what would they have been able to detect or confirm?
If answers can address what kinds of information mid-20th-century technology could have detected, what level of detail, how often various regions could be scanned etc - that would be wonderful. I would also love recommendations about other scientific developments from that era which might be relevant to this topic.
(Note: I'm doing this research for an alternate-history science fiction writing project. I've edited the question to only ask about real, historical science, since the goal is a narrative that accurately represents the past as much as possible.)


texts: How is the maximum density in a circumstellar disk determined?

texts:
I'm working with some equations to model the evolution of a circumstellar disk. One of the equations is
$$\rho(r)=Ce^{-\frac{(r-r_{peak})^2}{2 \sigma ^2}}$$
where $\rho$ is density, $r$ is the distance from the center, $C$ is a constant, $\sigma$ is one standard deviation, and $r_{peak}$ is the radius at which the density is at a maximum.

If the function was of the form
$$\rho(r)=Ce^{f(r)}$$
where $f(r)$ is a function of $r$, I could find the maximum easily by finding
$$\rho'(r)=Cf'(r)e^{f(r)}=0$$
and solving for $r$. However, this appears to be impossible in the current case because $\rho(r)_{peak}$ is already in the equation, at $r_{peak}$.

How is $r_{peak}$ determined in a given scenario? Is it determined experimentally?


texts: Understanding The Fluctuations In The CMB Maps

texts:
If I'm understanding this correctly the fluctuations in CMB are a result of the "last scatter" of photons when electrons joined together with nuclei that before this formed plasma. CMB is among other things used to see the early matter density fluctuations of the early universe, right?

So, how is this seen? Are the warmer parts in the maps where the matter is most dense? And are these spots warmer because more photons are coming from that spot, or because of some other reason?


texts: Flatness of celestial systems like galaxies and Solar System

texts: If we look around the cosmos, we will see most of the systems are just flat disks. Either it is solar system, galaxies, or the rings of Saturn. What is the matter, does it depend on how they were formed, or anything else?


texts: Could this three moons system be stable?

texts:
In my fictional world I have selected all parameters to maximize the Hills Sphere of my planet. The planet has three times Earth's mass and it is located in 2 AU from its star that's 1.4 times more massive than our Sun. I assume that there are more planets on the system but in such way to have minimum interaction with my planet.

I want to have three  moons on a 1:2:4 resonance. I also want:

the first one to be as large as our moon to the sky
the second 1.5 times larger
and the third, half our moon's size.

To achieve this I have assumed:

the first moon having 1.1 lunar radius,
the second 2.3
and the third 1.1.

Their orbits are 18, 36, 72 days.

I don't really care about their density and I assumed that their masses are

1 lunar mass (probably rocky)
3 lunar mass (water or ice world)
0.3 lunar mass (probably ice world)

 I guess that to be more stable i should have eccentricities smaller than 0.05 and inclination equal to zero.
 I am not sure about the spin of my moons, is there a chance being tidal locked?
This is my effort to achieve maximum stability of the system, what parameters should I reconsider to make it more stable? I am willing to change almost everything even the resonance, but I want them to be visible from my planet in similar way moon is to Earth.
Have in mind this question is about a fictional world for a fantasy story so even the minimum possibilities for that senario to happen is enough for me.
So any ideas?

P.S I have asked a similar question on worldbuild.stackexchange but they directed me also here for a better answer.


texts: What will "the Earth being consumed by the Sun" look like?

texts:
Sometime soon (astronomically speaking) the Sun will, as a main sequence star, expand into a red giant and, potentially, "consume the Earth".
If we take it as read that the surface of Sol will expand past the aphelion of Earth's orbit, what will Earth inside Sol entail, physically speaking, considering Earth as a rock, not an ecosystem, both as the surface of Sol expands past the orbit of Earth, and as the stellar envelope is ejected, leaving Sol as a white dwarf?


texts: Is there another explanation, apart from the doppler effect, for the redshift observed in distant galaxies?

texts: On episode 10 of the original Cosmos TV series, Carl Sagan mentions that some scientist don't agree that the redshift observed in distant galaxies is evidence of the big bang, stating that probably the doppler effect is not responsible for this effect. What other explanations are there?


texts: Is there any known asteroid or comet whose path goes through the Earth?

texts: Is there any known celestial body of at least 500 ft (150 m) diameter whose Minimum Orbit Intersection Distance (MOID) is within the Earth, meaning it's sure that it can impact the Earth? I know Comet 109P/(Kegler-)Swift-Tuttle has a very close MOID at 130,000 km (81,000 mi) and the Earth probably would be struck by particles from its tail if it came that close. And Apophis can come as close as 47,200 km (29,300 mi) but that's an asteroid (no tail). Are there any large bodies whose MOID is closer?


texts: Very bright star in the east at northern hemisphere. What is it?

texts:
From some time now (few weeks as far as I can remember), there is a very bright star in the eastern sky.

I first thought it was Venus, but according to this link, Venus is in the western sky and sets 2 hours after sunset. But this (very) bright star is still up even though it's midnight (6 hours after sunset).

Does anyone know what is this star (or planet) ?

Please excuse my silly question as I know nothing about astronomy.

Thanks.


texts: Is a General Relativity approximation available in Universe Sandbox (1 or 2)?

texts:
This answer points out that including General Relativity in a simulation can be computationally intensive and may not be present in Universe Sandbox.

However, answers to How to calculate the planets and moons beyond Newtons's gravitational force? discuss approximations that go most of the way toward including main GR effects in a typical solar system simulation where the effects are small.

Are there any options to "turn on" an approximation to GR in Universe Sandbox and/or Universe Sandbox 2? Or plans to add it later? And to double check, is it absolutely certain that only Newtonian mechanics is used currently for propagating orbits?


texts:
How do we calculate the mass of the universe taking into consideration dark matter?
Mass of visible matter can be computed by stellar method, but how do we calculate mass of dark matter which we don't see? Why do we assume that dark matter is more abundant than visible matter?


texts: Would a Phobos full impact make Mars temporary habitable?

texts: Due to tidal deceleration, Phobos is predicted to be destroyed by Mars in the next 30-50 million years either by collision or by breaking up in a planetary ring. If Phobos falls on Mars in one piece could the impact could be comparable with Chicxulub meteorite? Since Mars is smaller that Earth, would the effects of the impact cause a global restart of volcanic activity and temporary warm the planet? Would this be sufficient to make Mars more friendly to life for a few thousand years?


texts: Cosmology : Formula for the bias of galaxies

texts:
first I have posted originally on the physics stackexchange but I have not had answers about my question, so I try my luck on this specific forum (if forums leaders want to delete the original post on physics exchange, they can do it without problems, I don't want to make duplicates).
From this article : https://arxiv.org/pdf/1611.09787.pdf , I try to deduce the equation that my teacher told me which links 2 quantities :

the global number density of galaxies

the local number density of galaxies

the contrast of Dark matter density

The relation that I would like to find (the relation given by my teacher) is very simple :
$$N_{1} = n_{1} b_{1}\,\delta_{\text{DM}}\quad\quad(1)$$
where $$N_{1}$$ is the local number density of galaxies in Universe, $$n_{1}$$ is the global number density, $$b_{1}$$ is the bias (cosmological bias of galaxies) and $$\delta_{\text{DM}}$$ the contrast in dark matter density. When I say "local", I mean in the volume of scale that I consider (in a cluster of galaxies for example, doesn't it ?)
for the moment, I can't find this equation.
Into the article above, they define the bias by doing the relation $(1.1)$ (equation reference on the article) :
$$\delta_{g}(\vec{x}) = \dfrac{n_{g(\vec{x})}}{\overline{n_{g}}}-1 = b_{1}\,\delta_{\text{DM}}(\vec{x}) = b_{1}\big(\dfrac{\rho_{m}(\vec{x})}{\overline{\rho_{m}}}-1\big)\quad\quad(2)$$
with $b_{1}$ the bias.
As you can see, in this article, authors are reasoning with the contrast of density number of galaxies ($\delta_{g}(\vec{x}))$ and the contrast of matter density of Dark matter ($\delta_{\text{DM}}(\vec{x})$).
I tried to modify this equation $(2)$ to get $(1)$ but I am stuck by the following difference : on one side, one takes number densities and on the other one, they take contrasts of density (with contrast density number and Dark matter contrast).
Multiplying the both by the volume $V$ is not enough since there is the value "-1" in the definition of contrast : I don't know if I have to write :
$$\text{Global Number of galaxies} = \overline{n_{g}}\quad V$$
or
$$\text{Local Number of galaxies} = \overline{n_{g}}\quad V$$
???
I think that I have to use the following relations : $N_{g}\equiv N_{1}$ and $\overline{n_{g}}=n_{1}$ in the relation of my teacher but I am not sure.
Anyone could help me to find the equation (1) from the equation (2) of article cited ?
EDIT 1:
If I take the relation eq$(2)$, I can write :
$$n_{g(\vec{x})} = \overline{n_{g}}\,b_{1}\,\delta_{\text{DM}}+\overline{n_{g}}\quad\quad(3)$$
As you can see, $(3)$ is not equal to the equation $(1)$ that I would like to get (since a second term $\overline{n_{g}}$)
With the notations of the equation$(1)$, in order to be coherent, I think that I have to assimilate $N_{1}$ to $n_{g}(\vec{x})$ (local density) and $n_{1}$ to $\overline{n_{g}}$ (global or mean density).
How can I circumvent this issue about the presence of this second term into eq$(3)$ compared to eq$(1)$ ?
I am near from the equality between both, this is frustrating . Maybe it is a problem of convention about the factor $b_{1}$ called "bias" ?
Edit: This is also cross-posted at https://physics.stackexchange.com/questions/489437/demonstration-about-large-scale-bias-of-galaxies


texts: Why are Shack-Hartmann sensors so expensive (4k+ USD)?

texts:
Recently I am searching for a Shack-Hartmann wavefront sensor for university laboratory usage. I was expecting my target sensor to be cheap, which is:

Low spatial wavefront resolution (50 x 50) for beam measurement;
Low frame rate;
Portable design, and easy to use.

Most quotations are beyond my expectation. On ThorLabs it costs 4k USD (which is, "cheaper" compared to most competitors):
https://www.thorlabs.com/thorproduct.cfm?partnumber=WFS30-5C

Following reasons may to blame for this high price:

Expensive industrial sensors. But for https://www.flir.com/products/grasshopper3-usb3/?model=GS3-U3-14S5M-C it takes only 1.7k USD for a high performance sensor.
Microlens array (MLA) fabrication?
MLA-sensor alignment?
Software development? (SDK & GUI)

But I am not convinced by myself.

Any ideas why a Shack-Hartmann sensor is so expensive?


texts: Can we see evidence of sunspots on other stars

texts: Searching for exoplanets by measuring the change of a stars brightness when an exoplanet transits the star's disk also has the potential to detect exo-sunspots. Have they ever been detected?


texts: How to convert Log(g) vs T_eff to y vs (v-y)?

texts:
I have a plot in Log(g) vs T_eff coordinates from my stellar evolution model. The parameters of the model are M=0.8 M_solar, [Fe/H]=-1.19, Y=0.245. I want to convert it to the y vs (v-y) coordinates (Strömgren photometric system) in order to compare my model with observations of NGC 6752 globular cluster. How do I make this conversion?

This paper provides a table that includes the following columns: Log(g), T_eff, V, (B-V), (b-y), m1, c1.

I think I could use this if I can figure out how to calculate y and (v-y) from these data. Is it possible?


texts: How to calculate the orbital period and the density of a planetary body?

texts: I'm a student studying in 9th grade, I am making this fictional habitable planet named "Darwin B" for a planet making competition. It orbits a sun-like star at a distance of 1.15 AU or 172 million kilometres in a nearly circular orbit. Its rotational period is 19 hours, 38 minutes. Its mass is $6.15×10^{24}kg$ and its radius is about 6,743 kilometres. I have to calculate its orbital period and density but I'm weak in maths and don't know how to. Please help.


texts: I saw TWO moving stars going at the same speed! What could this be?

texts: This weekend me and my family took the opportunity to go camping. My dad and I went star watching because where we lived we didn’t get many stars. I was looking at the stars when I saw a bright star moving across the sky. It not as fast as a falling star or a meteor but it faster and brighter than airplane or a satellite. My dad and I kept watching the star when another star like thing moving as fast, in the same direction, but not as bright as the other star, started trailing the brighter star. We watched in disbelief as the two stars went across and soon behind a tree, out of our view. I can’t explain that. Can you?


texts:
Supposedly when the sun becomes a red giant, it will grow big enough to swallow Earth.
However, this requires the radius of the sun to expand by a factor of approximately 215×, meaning its volume would have to expand 10,000,000×.
Maybe it's just me, but something about this doesn't feel intuitive :-) especially given that the sun isn't gaining much mass. Exactly what about fusing elements heavier than hydrogen implies that the reactants and/or products take up 10,000,000× as much volume? Nobody ever explains this when explaining the stellar lifetime, and I don't understand why it should be the case. (In fact, I would have expected fusion to result in a decrease in volume, given that nuclei are combining...)
Edit:
It seems there are 2 types of red giants, some that occur during hydrogen fusion, some helium.
If the answer is different for these two types, I'd at least like to know the answer for the helium type (though of course I'd appreciate one that goes further and addresses both).


texts:
Using the standard formula for effective temperature of a planet, Venus has T(eff)=220K, Earth has T(eff)=255K and Mars has T(eff)=212K (using albedo for Venus a=0.7, Earth a=0.367, Mars a=0.25). Average surface temperatures of 730K, 288K and 218K respectively are easily explained by differences in greenhouse effect. The moon however with an albedo of a=0.1 should have a T(eff) of about 270K, but the average surface temperature is about 250K.
Here is where I got the T(eff) formula and here is where i got the average temperature of the moon. (the latter is a new citing and I have edited the mean temperature value) I have revised my planet temperatures using this reference.
What explains this negative difference?


texts:
We never see the dark side of the moon. It rotates so that the same side always faces us.

I heard two (dinner party) theories on this:
1. The moon was made by smashing out a chunk of the earth, and so somehow preserved some orbital rotation from the earth
2. The gravitational drag from the earth to the moon is lowest when the moon is in 'sync' to only show one face to the earth

I don't trust either of these.

My question is: "Why does the face of the moon 'sync' with the earth?"


texts: What do the lines in this Event Horizon Telescope and Global mm-VLBI Array infographic represent?

texts:
In the following EHT and GMVA diagram, what do the yellow and blue lines represent? I don't think they're physical communication lines (that would be unfeasible). Do they depict anything specific?

Source: ESO/O. Furtak


texts:
I recently won a Common 50mm refracting telescope with Solar Filter too.. I'm trying to Understand things I can skygaze with it. It's pretty basic and I should have known. Unfortunately, I don't possess this knowledge now. I had searched on various good websites which detailed and other requisite! Given, I stay in Navi-Mumbai, India, and less cloudy and less dusty area, what would my telescope's eye see?

And how to maintain and keep the Telescope in a good state.

What things would make me a better amateur-observational-astronomer?


texts: Which observatories help NASA find 5 new asteroids per day?

texts:
I just read the following in the NPR website news article NASA's New 'Intruder Alert' System Spots An Incoming Asteroid.

  NASA pays for several telescopes around the planet to scan the skies on a nightly basis, looking for these objects. "The NASA surveys are finding something like at least five asteroids every night," says astronomer Paul Chodas of JPL.

Which telescopes are the "several telescopes around the planet to scan the skies on a nightly basis" that NASA pays?

Also, is it really roughly five new actual asteroids identified per day (almost 2,000 per year), or is that 5 objects detected, and only some small fraction turn out to actually be unique, new objects?

5 per day sounds like a lot.


texts:
In a paper I'm writing, I've referenced an image I got from the NASA site at https://fermi.gsfc.nasa.gov/science/eteu/agn/ regarding the Unified Model of AGN. I captioned it

The AGN unified model, showing the BLR and NLR, as well as different types
based on the observational angle. Credit: Fermi and NASA:
https://fermi.gsfc.nasa.gov/science/eteu/agn/

but my supervisor said that I should use proper referencing (I see that the image seems to be derived from this paper). The image from the NASA site is much clearer and easier to understand, and I'd much rather use it for clarity's sake.

What's the proper way to reference an image like this, which is derived from a paper?

Maybe I should just use the original image?

Or does anyone know of a better quality one from another paper, similar to the one from the NASA site?

EDIT: I should mention that this is a follow-up question to one I posted here.


texts: What will be Supernova SN 2014J's peak brightness?

texts:
What is the expected maximum apparent magnitude of supernova SN 2014J?
When will it attain peak brightness?


texts:
I have not been able to find an answer to my question through google, and therefore ask it on this medium.

Do brown dwarfs have spots and faculae similar to other heavier stars?


texts: Is the atmosphere of neutron star solid, liquid or gaseous state?

texts: Neutron star has a thin layer of atmosphere consist mostly of hydrogen and helium and is about less than a metre thick on average, but what wasn't stated in many articles I read is the state of matter of the atmosphere.


texts: Abundance of penitente ice formations in the solar system

texts:
When researching for my question on extraterresitral snow, I learned about a rare snow formation called penitente which seems to be found on various (dwarf) planets, like Earth, Mars, and Pluto. Interestingly, these spike-like structures are an important place for life in extreme environments (on Earth) as shown by a study 2019 of penitentes in the Adnes, but this is not what I after.
My question: How abundant are penitentes in our solar system? Are they any estimates in terms of area covered or total ice volume present as penitentes per planet? I would also be interested in actual pictures of extraterrestrial nieves penitentes (including the image source).
How it looks like

Penitentes of the Atacama Desert, cropped from http://www.eso.org/public/images/potw1522a/
References
The Reference list of the Wikipedia article already offers a nice reading list, but I feel the list is far from exhaustive. What I found worth bookmarking:

A blog by Yurong Yu from February 2017 served me as a starting point.
There is an 2019 article by T. Giang Nguyen, Christina L.Smith, Alexandra Innanen and John E. Moores called
Simulating the formation of Martian penitentes
The same John E. Moores published meeting notes called Penitentes at Tartarus Dorsa, Pluto.


texts: Why the light curve goes down when the planet is behind the star?

texts:
There is a video explaing the transiting exoplanet light curve — https://www.youtube.com/watch?v=RrusIZaWDW8

It is clear to my why the curve goes down when the planet is between the observer and the star, but I don't understand why the curve goes a little bit down when the the planet is behind the star. I was expected that this should not change the light curve at all.


texts: How much more massive would Pluto have to be to clear its neighborhood?


texts: Globular cluster star density as a function of distance from the center

texts:
Context: I want to simulate globular clusters in a simple way, just to display the positions of stars.

Assuming isotropy, what would be a reasonable model of the stellar number density as a function or r, the distance from the center of the cluster?

I'm assuming that such a model would have some free parameters as well.


texts: Trying to use Canon 60D DSLR with Skywatcher 130P. Can't focus image

texts:
I have a Skywatcher Heritage 130P FlexTube Dobsonian Telescope. It works well, I can see lovely stars, planets, moons of Jupiter etc.  It came with two eye pieces (1.25 inch): 10 mm and 25 mm.
I also have a Canon 60D camera.
I've been trying to attach the camera to the telescope using a Celestron 93640 1.25 Inch Universal Barlow and T-Adaptor which I originally bought for a Celestron Travelscope.
I have never been able to get any kind of decent image out of the camera when attached to either  telescope. Am I doing something daft? (This is all new to me).
Steps I take:

Set up and focus the Skywatcher scope on some nice stars using the 25mm eye piece.
Remove the eyepiece.
Insert the camera with Barlow and T ring where the eye piece was.
Diddle around with the focus. Try and take some photos. Get nothing remotely in focus, not ever a blurry star.

I suspect I have the same issue as described in Astrophotography - Unable to achieve focus with current setup If so, please could someone advise the adapter I would need for the combination of camera and scope I have? I'd prefer to use the DSLR than buy a digital camera if possible. (Many thanks, and sorry if this is a daft question.)


texts: What do the fusing 'onion layers' of a pre-supernova star look like to scale?

texts:
I'm sure we've all seen the diagrams of various layers of element fusion from hydrogen to silicon in a star that's just about to go supernova.

(Picture from courses.lumenlearning.com)

I suspect these pictures grossly exaggerate the relative radii at which these fusing layers occur for the sake of readability. What would a more accurate to-scale picture of these fusing layers look like?

Now, I assume the answer will vary significantly based on the mass of the star, and I suspect in certain mass regimes some layers will not be fusing at all. I can think of a few other variables that may affect the answer too.

So in CYA fashion, I'll leave it up to the answerer to identify specific illustrative or interesting cases, as I'm not after a specific answer but rather a general feel for how large some of the layers are in comparison to the others.


texts: What is the farthest we can see into the universe?

texts:
Is there a limit in which the mass in space obscure the ability to detect anything farther even as technology progresses?

Related:

Can the range of the observable universe be extended through an intermediary?
Which galaxy is receding from the Milky Way the fastest? What is known of the mechanism behind its recession?


texts: Can I point to somewhere in the sky where the Big Bang happened? A pathway so to speak on where we (and our progenitor elements) came from?


texts: If a black hole has a mass similar to a star, why does it have a different gravitational pull?

texts: Say there is a black hole with 1 solar mass, having the sun's mass, it would have the same gravity as the sun, right? But it still has an event horizon, so why does it have such a strong gravitational pull despite its mass?


texts: What is the difference between Terrestrial Time (TT) and International Atomic Time (TAI)?

texts:
The equation which relates Terrestrial [Dynamical] Time (TT, or TDT) and International Atomic Time (TAI) is the following, according to the U.S. Naval Observatory:

$TDT = TAI + 32.184$

As I have understood, both TT and TAI are continuous time systems (differently from UTC, which has leap-second gaps) and described by the same unit (the SI second), but counted from a different epoch (the origin T0 of one is 32.184 seconds after the other).

It appears that both time systems are essentially the same. So what is the actual difference between them? If the difference between them is the nature of how they are computed (e.g., TT is a theoretical timescale; and TAI is a statistical timescale based on several clocks), why the 32.184 shift?


texts:
I "missed the train" on the Starlink satellites last year, and now it looks like they've separated, and the conservative voices convinced Space X to darken new ones with visors or paint.
Is it still possible to see 2 or more satellites moving in the same direction with the naked eye? (Actually, any celestial bodies visibly moving in the same direction will do.)
I'm located in California and I tried Find Starlink and the Heavens Above app but wasn't able to see anything.


texts: Galactic revolutions relative to the age of the universe

texts: If the universe is only 40 to 50 galactic revolutions old, how has it acquired its present state so "quickly" relative to this time scale?


texts: Angular speed of a planet across the sky

texts:
Assuming circular orbit, the evaluation of the angular speed of a planet around the Sun, just requires its orbital period. Similarly, to calculate its angular speed relative to the Earth (also seen from Sun and considering the approximate circular orbit), its synodic period is needed.

The question is: what is the angular speed of an interior planet across the sky in the conjunction with Sun and how can I deduce its formula?


texts:
While searching for different methods of speed of light measurement, i came across one of the method of fizeau discussed below which i cannot fully understand.

In short,in Fizeau’s apparatus, a
beam of light was shone between the teeth of
a rapidly rotating toothed wheel, so the
“lantern” was constantly being covered
and uncovered. Fizeau had a
mirror, reflecting the beam back, where
it passed a second time between the
teeth of the wheel.

I do understand the idea i.e. to measure the time during the course of light "from the wheel to mirror and then back to the mirror".But where are the mirror,lantern and wheel  located? What kind of wheel is that? Does light pass through the holes (or teeth) in the wheel or light gets refracted through the wheel? Finally if a group of light particles goes through one of the hole (or teeth),then how do we know that the same group of light particles came back through which hole (or teeth) after its journey,since there are soo many light particles leaving and entering different holes (or teeth)?


texts: Is there a term for a horizontal moon crescent

texts:
I seen the other day the moon lit directly below or towards my horizon perfectly.

My question, is there a term for when a moon's crescent is aligned with the viewer's horizon?


texts: We have just passed one of the solstices and are approaching aphelion.  The two events are close but not simultaneous.  There is no very obvious (to me) reason why the two should coincide or be close.  Is there a less obvious reason why they are close?  If so, is the gap closing, growing, or remaining approximately constant?


texts: How large could gravitational waves get and what effect could they have on us?

texts: The waves detected by LIGO were basically subatomic distortions of space-time. Could there be much greater distortions closer to a source. Would they become perceptible to a human in any way and what would it do to us?


texts: Do sungrazing comets leave a field of meteoroids near the Sun?

texts: A large fraction (a third?) of all comets found (with observational bias thanks to SOHO) have been sungrazers, breaking up with perihelion of a few of Sun's radius. Is there reason to believe that this replenishes some kind of vulcanoid population of small objects? Or would they evaporate and be blown away by the Solar wind, or continue in their comet's orbit, or be ejected by Mercury's eccentricity? Would the constant replenishment, even now, not billions of years ago, help keep a population there in spite of all above?


texts: According to wikipedia (https://en.wikipedia.org/wiki/Coma_Cluster), Coma cluster contains over 1000 galaxies. Is there any upto date catalogue of these galaxies with the following given parameters (Ra, Dec, z, magnitude, position angle, major diameter, minor diameter).


texts: Is it coincident that all planets with tight orbit are tidally locked to their parent body?

texts: I know mercury is tidal locked to our Sun like our Moon is to Earth, is it merely coincident or there is some sciences behind it? Some man-made communication satellites are also tidal locked to Earth! I think it is intentional so that there is minimum interruption for signal transmission, they used thrusters and gyro to help them to always point toward Earth.


texts: First spectroscopic detection of Uranus' rotation?

texts:
As an undergraduate exercise quite a while ago, we placed the equator of Jupiter's disk across the slit of a high resolution grating spectrometer and then measured the tilt of the resulting line in order to estimate Jupiter's rotational velocity.

When was an experiment of this type first performed for Uranus?
What was the result? Did it make sense at the time? Was Uranus' 98 degree axis tilt appreciated?

Related: Uranus' axis of rotation-when discovered?


texts: How to know that the 21cm Hydrogen line is the actual emission and not any other redshifted line?

texts: Is it possible that we mistake some other wavelength emission to be 21 cm because of redshift?


texts:
Would anyone be able to reference the galaxy in the background of this image of the Ring Nebula:

It's obviously not a Messier object, I'm more confident in it being in the New General Catalogue.


texts: What is the difference between these various Angular Resolution formulas?

texts:
I have recently learned about angular resolution and its dependence on the ratio λ/D. There seem to be three different equations for angular resolution that I have come across. One of these has the number 1.22 (radians), while the other contains the number 206,000 (arcseconds). A third equation states that angular resolution can be found just with the ratio λ/D. I have attached a picture of these three different equations - please pardon the bad merging!
Naturally, this has caused some confusion on what situations each of these equations apply to. Would one of these be more relevant to an interferometer — while another is for a single telescope? I'd like to share that these equations come from the textbooks "Foundations of Astrophysics" (Ryden & Peterson) and "Understanding Our Universe" (Palen, Kay & Blumenthal). If anyone has come across these equations, I'd be grateful for some advice on how to distinguish them (i.e. what situations or conditions they apply to).

$$\theta_\text{min}[\text{rad}] = 1.22 \frac{\lambda}{D}$$
$$\theta = 2.06 \times 10^5 \frac{\lambda}{D}~\text{arcseconds}$$
A telescope's resolution is determined by the ratio $\lambda/D$.


texts: Cubic spline interpolation in light curve of pulsating stars

texts:
Why is cubic spline interpolation used to analyze light curve of pulsating stars? Is there any scientific justification of such usages?
One example of using the cubic spline interpolation is here, section 4.


texts: How is the term "solar system" defined? Could confirmation of a new planet lead to a change in this definition?

texts:
Voyager 1 (https://en.wikipedia.org/wiki/Voyager_1) is approximately 140 AU away, and is considered to have left the solar system. Now, the layman's definition of solar system provided by Google is:

  ...the collection of eight planets and their moons in orbit around the
  sun, together with smaller bodies in the form of asteroids,
  meteoroids, and comets.

And from Wikipedia:

  The Solar System[a] is the gravitationally bound system of the Sun and
  the objects that orbit it, either directly or indirectly,[b] including
  the eight planets and five dwarf planets as defined by the
  International Astronomical Union (IAU).

At this point, Voyager 1 is well beyond the aphelion of all known planets, so according to the definitions above, so at least colloquially, saying that it has left the Solar System seems to make sense. However, according to Wikipedia, it was only officially considered to have left the Solar System once it left the heliopause. This is not really in conflict with the above definitions, as, to my knowledge, the heliopause extends beyond the orbit of all the original 8/9 planets at all times. The Encyclopedia Britannica (https://www.britannica.com/science/heliopause) states that the heliopause
extends for 123AU.

However, the hypothesised Planet 9/X is conjectured to have an aphelion of approximately 1,200 AU and a Perihelion of 200 AU (https://en.wikipedia.org/wiki/Planet_Nine). This would seem to bring the two competing definitions of "solar system" discussed here into sharp conflict.

Which definition would likely win out? Could the confirmation of such a planet force a reckoning on whether Voyager 1 has truly left the solar system?

Edit:
I wanted to add a more authoritative source that says Voyager 1 has indeed left the solar system.

  Voyager 1 is the first man-made object to leave our solar system and pass into interstellar space. Scientists confirmed this finding a year later after studying Voyager’s data, which showed clear changes in the plasma or ionized gas right outside of the solar bubble.

https://www.nasa.gov/directorates/heo/scan/images/history/August2012_2.html


texts: How large were the deviations of Uranus's positon from predicted which led to the discovery of Neptune?

texts:
After Uranus was discovered and its orbit calculated, its future orbit was calculated, and its future positions as seen from Earth were calculated.
And observers of Uranus began to notice that Uranus was deviating from its calculated positions.  Eventually the possibility that an undiscovered planet was perturbing the orbit of Uranus was used by Adams and Le Verrier to calculate the orbit of Neptune and led to the discovery of Neptune.
So how large were the deviations in the apparent position of Uranus that were used to calculate the orbit and apparent position of Neptune?


texts: Frames per second capabilities cameras currently installed in Modern Telescopes?

texts:
What are the present real-time capabilities of various modern telescopes as video cameras?  How do these compare with the capabilities various of space telescopes?

For example, if it was known that there would soon be a Saturn-comet impact (akin to the Shoe-Maker levy impact into Jupiter), what would the "frames per second" of the captured imagery be, regardless of imaging resolution?

Video of the Shoemaker-Levy impact: https://youtu.be/p7RP2SW_gSw?t=92.  Is this video presented in real time?

Animation of the Shoemaker-Levy impact:

https://en.wikipedia.org/wiki/Comet_Shoemaker%E2%80%93Levy_9#/media/File:Max_Planck_Institute_Shoemaker%E2%80%93Levy_9.gif

Live Streaming Jupiter (from Earth):

https://www.youtube.com/watch?v=ILh4lWHi_ag

Shoemaker-Levy image:

https://en.wikipedia.org/wiki/Comet_Shoemaker%E2%80%93Levy_9

ISS Live Stream [for comparison]:

https://www.youtube.com/watch?v=EEIk7gwjgIM


texts: Could a planet like the Earth or any other explode or be destroyed and did we see a planet explosion with Hubble?


texts: What is the exact time of sun stay in each zodiac every year on sidereal year duration?

texts:
I tried to reverse engineer from astro-chart times at which sun stays in a zodiac.
Here is the list.

Sun_Stay_zodiac  timeofstay_sec   timeofstay_days
  1  Aries         2672700     30 day 22 hour 00 minute
  2  Taurus        2714700     31 day 10 hour 05 minute
  3  Gemini        2734080     31 day 15 hour 28 minute
  4  Cancer        2719440     31 day 11 hour 24 minute
  5  Leo           2680080     31 day 00 hour 28 minute
  6  Virgo         2586960     29 day 22 hour 00 minute
  7  Libra         2626020     30 day 09 hour 27 minute
  8  Scorpio       2548020     29 day 11 hour 47 minute
  9  Sagittarius   2533080     29 day 07 hour 38 minute
 10  Capricorn     2544240     29 day 10 hour 44 minute
 11  Aquarius      2576520     29 day 19 hour 42 minute
 12  Pisces        2622540     30 day 22 hour 25 minute

Since the orbit of earth is elliptical and therefore also variation is orbital speed the stay of sun in each zodiac is different i.e Maximum at Gemini and after 6 months minimum at Sagittarius. However when I add them up sums up to 31558380 seconds which is 230.495 seconds than time in seconds in sidereal year ie 31558149.504 Now what I am looking for is more exact time duration of sun stay in each zodiac that includes seconds. I hope this data that I have in my question will help others too.


texts: Why at night the light of all stars get in small amount to earth?

texts: What i mean, why we still get dark if all the stars are sending their light to earth.


texts: Could a moon be passed from one planet's orbit to another?

texts: Like if you had two planets that had a close orbital path could the moon be pulled away from the first planet?


texts: Is it because there isn't life to exhale enough oxygen and carbon dioxide? The size of Mars is about the same as Earth, so isn't there enough gravity to hold enough air?


texts: How far out would we need to travel to measure the "constant observer"?

texts:
One hypothetical tenet of much astrophysics is that the Universe is uniform on large scales.  This actually hinges on many deep theoretical results from astrophysics, since what we actually observe is that the Earth is at the center of the observable universe, and that stuff near the edges of the observable universe is quite different than stuff near the center.  However, these theoretical results could be verified by, e.g. traveling to a different point in the observable universe and observing the universe from that point.

Unfortunately, we are currently constrained to observation from Earth, so this concept of the "constant observer" is difficult to test directly.  At current technological levels, how far would we need to travel away from the Earth to perform an experiment directly verifying the "constant observer" effect?


texts: How to calculate B-V colour index value percentage difference

texts:
I need to calculate a percentage difference of a B-V colour index between its estimated and actual value. So I tried doing this by difference between values/actual value x 100. However as B-V values can be on either side of zero this did not give representative answers.

So if anyone knows a way of calculating percentages of values close to zero or on either side of zero their help would be appreciated.

Thanks


texts: Astronomical resources helpful for creating an animation in the browser of the movements of an astronomical object in the sky?

texts:
Background
I would like to create a website that shows an animation of the path of Sagittarius A* (or any other object in space) during one day for a specific location. So that the user can see where and when Sagittarius A* is rising, what path across the sky it takes and where and when it is setting.
Similar to this:

I know the animation in the YouTube video was made with "Stellarium Web" and I could make something like that with Stellarium for Sagittarius A*. But I would like to have it interactive on a website where the user only has to enter his current location and then would see the animation of the path for his location.
Question
Where can I find astronomical resources for getting the data necessary to make such a plot in a way useful for a website? It would have to be something I could call from, or incorporate into a website, not a stand-alone program.


texts: How do we find the king model of a globular cluster?

texts:
So we have data for a globular cluster M13. we have plotted the radius in pixels against the surface brightness in magnitudes per square arcsec.

We have to find the king model for this. We dont exactly know how to do that. We think we just have to set the units to solar luminosities and then plot it again but if that is the case we dont really know what our graph tells us that the first graph did not tell us. So we think we are doing that wrong. If we are acually doing it right we dont know what this tells us. Can somebody help?


texts: Are all models of ocean planets theoretically cloud covered?

texts: Take for example an ocean planet slightly smaller than the Earth. Let's say it has a planet-wide ocean that is on the average 10 miles deep. Say for argument sake, that there is a molten, rotating outer core producing a magnetic field and that there are plate tectonics (although not to the same degree as Earth). Also, imagine that there are volcanoes that reach as high as 5 miles below sea level. The planet's effective temperature is similar to Earth (not counting greenhouse). Would such a world without land always be covered with clouds? Would it necessarily have a runaway greenhouse? How about such a world with no vulcanism?


texts: How accurate and complete is the Hubble sequence diagram?

texts:
The Hubble sequence sorts and characterizes or classifies galaxies by their shape. It was suggested by Edwin Hubble back in 1926.

One drawback, or criticism, of this scheme is that Hubble had a limited data set of galaxies at hand, compared to today's data. He "only" had a few hundred galaxies in visible light from which he could build the sequence.

So, adding today's knowledge of way more galaxies imaged in all different kinds of spectra how correct is Hubble's scheme? Is it skewed because of this originally limited selection of data? Are there other shapes missing in the sequence?


texts: Is there a possibility that a white dwarf can turn into a neutron star or a black hole?

texts: I know that a white dwarf is mainly balanced by electron degeneracy pressure and that if it gains more than 1.4 solar masses from any source (such as a companion star or a collision), it explodes as a type I supernova. But is there any possibility that a white dwarf can turn into a neutron star (or possibly a black hole)?


texts: Current limitations in radio-astronomical spectrometry analysis of local interstellar cloud's hyperfine structures

texts:
I've been trying to wrap my head around capabilities of current radio-astronomical spectrometry technology to isolate not too distant tenuous sources, say, chemical composition and density of the the interstellar medium right in the neighborhood of our Solar system. In essence, what is our local interstellar cloud made out of and how much of it is there:

We've had much success so far in isolating distant radio wave sources and analyzing their hyperfine, i.e. molecular level composition. Same goes even for interstellar medium composition down to molecular ions of diazenylium (N2H+) that is surrogate to the presence of unobservable, nonpolar N2.

For example, P. Caselli et al. from the Harvard-Smithsonian Center for Astrophysics presented a paper (PDF) on J = 1 → 0 transition of N2H+ at 93 GHz toward the quiescent low-mass cloud core L1512 in Taurus, back in 1995. That is 18 years ago!

What I don't understand is, what's preventing us, with current technology, from isolating local low-mass formations in the radio wave spectra from its background noise and analyzing its hyperfine structure through radio-astronomical spectrometry? Is the signal to noise ratio simply unworkable and the density of the local cloud too thin to filter it out from its background? Or have such observations actually already been done, and we already have molecular data on the local cloud that I'm not aware of?


texts:
I'm in doubt whether I should put this in the astronomy or rather the coding exchange, but since my interest goes out to the former I will try my luck here.

Suppose I have an image with a certain pixel scale $x$. I would like to rebin this to a larger pixel scale, $y$, but this is not an integer multiple - so I can't say, for instance, that every group of 4 pixels in the original grid will now be one big pixel in the new grid, and average their flux.
Rather, I'd like something like i.e. $y=2.3\cdot x$.

Is this routinely done in the astronomical community -- and if so, how?


texts: Is there really a blood wolf moon this month?

texts: Just wondering so I can see it because I don't know if  it is happening this month or not?


texts:
I know stars explode because of the fuel causing a fusion compound pushing it apart and the fuel runs out and it 'bounces' for lack of a better term.

Given the fact that more massive stars are supposed to explode more quickly: why doesn't the Sun explode considering its massive size?


texts: Where can I find a database of galactic spectra?

texts: I was wondering where spectra of galaxies at various redshifts are to be found. I'm looking for ones that can be used to find the recessional velocity of the galaxy and eventually the Hubble Constant. Here's one I found:


texts: What is the tidal radius of the Sirius system?

texts:
The Fomalhaut system is a trinary system, with Fomalhaut A (1.9 M), Fomalhaut B (0.7M) and Fomalhaut C (0.2M). Fomalhaut C is 2.5 light years from Fomalhaut A, and 3.2 light years away from Fomalhaut B. The tidal radius of the Fomalhaut system is 6.2 light years.

The Sirius system (Sirius A, 2M, Sirius B, 0.9M) is roughly 1.1x more massive than Fomalhaut A and B. What would be some ways to calculate a tidal radius of the Sirius sytem, to get an estimate of at what distance other stars could orbit the Sirius system?


texts: Why does the planet Saturn have numerous (62) moons compared to the rest of the planets in the Solar System?

texts:
Saturn is a gas giant like Jupiter. It has everything from

  tiny moonlets less than 1 kilometer across to the enormous Titan, which is larger than the planet Mercury. Saturn has 62 moons with confirmed orbits.

  Wikipedia

Why does Saturn have more moons?


texts: In what units to quote the thermal Blackbody temperature

texts: Just a quick question relating to the thermal Blackbody temperature of a celestial object. In the c.g.s regime of astrophysics, is it more 'sophisticated' to quote the thermal Blackbody temperature in units of Kelvin ($\mathrm{K}$) or in units of $\mathrm{keV}$? That is to say to use $k_{B}T_{\mathrm{bb}}$ and convert to an energy?


texts: Meaning of critical density in radiative transfer

texts:
I do not understand completely the meaning of critical density when talking about atomic or molecular line emission in astrophysical gas clouds: I know that it is way more complex than this, but just taking the two levels approximation (i.e. an atom/molecule with two quantum levels, 1 and 2, where 2 is the upper one) the critical density would be
$n_c = A_{12}/\gamma_{12}$, where $A_{12}$ is the Einstein coefficient for spontaneous radiative emission (i.e. de-excitation) and $\gamma_{12}$ the collisional de-excitation probability.

It seems to me that if the gas density is $n < n_c$ the radiative de-excitations dominate, so I would expect to see the photons; conversely if $n>n_c$ collisions dominate, so the particles will lose energy through collisions before having enough time to spontaneously decay and emit photons.

This argument of mine, however, seems to be wrong, because everybody use $n_c$ exactly in the opposite way: I see the photons only if $n>n_c$. I would expect that for $n \ll n_c$ the particles are experiencing almost no collisions, so neither collisional excitation, and they do not decay because they are not excited in the first place; but I feel that something wrong has been written in the previous lines, so I would like to read someone's opinion about it before to make even more pie in the sky conjectures.


texts: How can I calculate when the new moon will be observed from a particular meridian?

texts:
I have read that Chinese months start on the day on which the new moon is observed from the 120th meridian east.  From this I am assuming that the new moon is observable from there each time and that it doesn't matter where on the meridian you are (although I suppose it's implied you're in the northern hemisphere).

How can I calculate the time at which the moon is new from a particular meridian?


texts: A planet (as well as a dwarf planet) must, according to the IAU definition, have sufficient mass to assume hydrostatic equilibrium (a nearly round shape). Does it mean they would break apart or explode if gravity vanishes, unlike small bodies like comets or minor planets, whose integrity doesn't depend on gravity?


texts: Why does gravity increase in star formation?

texts:
When a star ignites ( ie. fusion starts ), the star maintains its form by balancing gravity's inward pressure, and radiation's outward pressure.

I get that the fusion of hydrogen atoms releases energy... fine...

How does gravity keep it together if the mass is lessening as a result of fusion( mass being converted into energy from fusion) while gravity is weakening( as mass lessens )?

Wouldn't the radiation overpower the force of gravity and tear the star apart?


texts: I have always seen answers to this point to measurements of how "flat" local geometry is, but my problem is, thats space not spacetime. I would say its safe to say that the geometry of space in our universe is indeed euclidean, but space isn't over time. The further I look, space should be less, the farther I go, space should be more. To me this seems as if spacetime is hyperbolic along the temporal axis, and that every "instance" cross-section is euclidean. So that ultimately is the question, is spacetime hyperbolic?


texts: What planets and moons have we seeded with life (if any)?

texts:
Which planets and moons have we accidentally (or purposely) seeded with life?  I would expect this to be any planet or moon we have sent an unsterilized (or not fully sterilized) lander to, or crashed anything into.

I am speaking largely of bacteria and other microorganisms that might have hitched a ride on our spacecraft, or any life that was deliberately taken to and left on another world (which I am not aware).  I know we went to the moon, we at the very least "seeded the moon" temporarily with life (humans) and probably bacteria on the space suits / lander / equipment we sent.

I wondering where we have "seeded" / brought life (even if it doesn't / didn't survive there).


texts: Oxygen in a methane atmosphere akin to methane in oxygen atmosphere?

texts:
If we assume a planet the size of Earth with an equally dense atmosphere of nitrogen and methane, rather than oxygen, and it's in the goldilocks zone. Would a hypothetical alien species be able to use oxygen for lanterns and stoves the same way we use methane (or natural gas, etc)?

If so, what primary differences in ratios and/or densities of gasses would have to exist? If not, why not?


texts: Why would discovering life on another planet be important/matter to us?

texts:
I had a funny thought, and would like to pose it to you:

When I was a kid, reading any kind of astronomy or similar books would all say that there was no other life in the universe, and made it pretty clear that anyone thinking there could be was probably off his rocker.

Nowadays, it seems this has become a very important question for astronomers and other scientists, a new deep-seated belief that must now be proven. Some seem to think it would be the most important discovery of all time.

I'm wondering why?

My guess is, if we did 'discover' life on another planet, it'll make the news for a bit, and then the average person will go back to their every day life without much more thought about it.

For me, I would think this is great, only insofar as it means a possible new source of food (i.e., some delicious exotic cuisine assuming it's edible, and possibly even some place to go conquer and colonize if there was any profitability/money in doing so).

For some scientists, though, I get the sense it's more of an axe to grind with the long-dreary he-said she-said argument of science vs. religion.

Why do you think it would matter?

My question assumes any life discovered is probably not going to be very 'interesting' life, at least nothing we're going to have a conversation with.


texts: Are there observable changes in a star about to become supernova, minutes or hours before the explosion?

texts:
I am writing a science fiction novel, where a ship is stranded in a single star system (a red supergiant). One of the plot points is the star becoming supernova in several hours, so the characters have to fix their ship before that happens.

I have basic knowledge of how it works: Iron generated from nuclear fusion gets accumulated in the core, until it reaches a point when iron fusion starts. As iron fusion is an endothermic reaction, the core is no longer able to generate enough energy to hold against its own gravity and external layers pressure, so it collapses, and explode.

I have read that once the iron fusion starts inside the core, the collapse occurs within minutes, that the collapse itself lasts a few seconds (even less than a second), and that the shockwave takes several hours to reach the surface. Is all that correct?

The thing is that I need the characters to bee able to predict the explosion in a short term. A few hours or even minutes. It would be great if they could be aware of the core collapse and start a countdown.

So, are there any external cue of these events, like changes in luminosity or color? Does the star spectrum change when iron fusion starts, or when the core collapses? I know that the core collapse generates a huge amount of neutrinos. Is this amount so intense that it can be easily detectable? (that is, without a huge detector in an underground facility). Can the amount of iron in the core be estimated from tha star spectrum and size, so the aproximate time of the collapse could be predicted?


texts: What is the advantage of taking monochrome photographs in standard lenses

texts:
I've readen "When photographing faint objects monochrome photos will resolve more than RGB pics"
Is this really a thing? If yes, how?


texts: Turn celestial coordinates into light years

texts: I have a question about celestial coordinates. I have been working on a calculator that takes in different stars and tells you how far apart they are, and their coordinates (x, y, z). But I didn't realize there was no such system. Is there any way to turn celestial coordinates into a coordinate plane, i.e. (x [ly away from earth], y [ly away from earth], z [ly away from earth])?


texts: Why is the speed of light 299,792,458 meters/sec?

texts:
Ok, I am majoring in physics (4th year) and I never understood this fundamental (kinda) question. Maybe I haven't explored it enough.

For example, why does it take 8min20sec for the light from the sun to get to us?
I know the answer to this question on a 'surface' scale. The sun is 1AU away, c=3E8 m/s, and d=v/t to get approx 8 min 20 sec.

my question is on a deeper level.

Say you could "ride a photon" I KNOW THIS IS IMPOSSIBLE, but just say you could. Or a better question: what does a photon experience? The photon, as per my understanding, would leave the sun and (if it is on the right trajectory) hit the Earth instantaneously. A photon leaving Alpha Centauri would see the universe all at once, in a infinitesimal small unit of time (if directed out to space).

If a photon sees everything all at once, why do we perceive it to have a speed? I am sure this has something to do with frames of reference, special relatively, Lorentz transforms? but just seems strange. why is the speed of light finite to us... if it was infinite would this be problematic?


texts: How can I determine if a typical low-end telescope is a refractor or reflector?

texts: Is there a way to distinguish (with no manual, product info etc)?


texts: When was Hubble tension first noticed? When was this term first used? Who used this term for the first time?


texts: Is it possible to get the distance to a star in the IPHAS DR2 catalog?

texts: I'm looking at the IPHAS DR2 catalog. It has the Right Ascension and Declination but I'm not seeing any distance information (lightyears or parsecs). I'm wondering, given the columns in the dataset, if the distance can be calculated.


texts: Can an astronaut ever reach a Black Hole theoretically?

texts:
Ok, bear with me, while I try to explain the question I have.

I read that time slows as we approach a black hole.

So, suppose,theoretically, as astronaut is approaching a Black Hole. And he gets near enough now that time starts slowing down.

If you remember this equation: v=d/t  So, at the moment, t is going infinity, so it means
V is going zero.
So the astronaut can never the reach the Black Hole because time is slowing down and its taking longer and longer for him to approach the Black Hole.

But then again, we know, a Black Hole sucks everything and that light cannot even escape it.

So, what's happening to the equation above then? [ this time it means v is going infinity and so, t is almost zero], so the astronaut will be sucked almost immediately.

Which one is the correct theoretical answer?


texts: If the atmosphere on Mars was once much thicker, how was it likely lost?  Was it due to interaction with the solar wind, the small size of the planet, both, or something else, and approximately how long did it take to reach its current thickness?


texts: Classical Mechanics : Interaction Sun Radiation Pressure - Gravitation

texts:
I have a theoretical problem that merges the effects of a gravitational field and the Sun radiation pressure. The problem goes as follows:

A spacecraft orbits the Sun in absence of any other body interaction. The spacecraft surface is not negligible compared to its mass and it
faces the Sun with the same orientation at all times. Does the
trajectory satisfy the conditions of a Keplerian motion (i.e. is the
trajectory a conic section?) Same question when the Sun is not a
sphere but a flat disc whose axis is pointing always in the direction
of the spacecraft.

My answer is that in both cases we can consider that the spacecraft will move in a conic section. If you consider that SRP is always acting in the opposite direction of the gravitational pull of the Sun, both forces would form a central vector force field, where the gravitational force would be decreased by the SRP. If the motion described by the S/C only considering gravity is a conic section I do not see any reason why it should be different in that case.
For the case of the Sun as a flat disc, considering that it would be pointed always to the S/C, the force is again central (although the potential would not be circular).
I have not found a better explanation for this, although I feel there is some kind of theoretical insight that I might be missing.
Thanks for the help!


texts: What happens to the Gas Pressure when working out the Eddington Luminosity?

texts:
I am looking at how the Eddington Luminosity is derived and I do no understand why we only care about $P_{rad}$?

When working out $L_{Edd}$ you take the ratio between  $dP_{rad}/dr$ and $dP_{hydrostatic}/dr$ which must be less or equal to than 1. If $P_{hydro} = P_{gas} + P_{rad}$ why don't we take $P_{gas}$ into account anywhere?

For reference:

$$L_{Edd} = \frac{4\pi c G M}{\kappa}$$


texts:
A full disclosure to begin with: I'm a PhD student in mathematics and while I understand most of standard cosmological-astronomical terms and I've followed a one semester course on cosmology, I don't really have a deep comprehension of the well known material neither on cosmology or astronomy.

I've recently heard about the possibility of a "redshift quantization". As I understood this is not a mainstream result, which means that as 2007 was still quite debated. Anyway I couldn't understand if it really was ruled out by some more recent observations or if it's a still ongoing idea. But to be more specific what I really don't understand are the implications of this result and if this result would invalidate some current cosmological theory and why. I've heard it might have some implication on Quasar formation and change the current timeline of the Big-bang but didn't get how nor why.

Can anybody give me some hints?


texts: How to calculate shape of orbit when projected onto the plane of the ecliptic

texts: I'm trying to generate an 'overhead' view of the orbits of the moons of Neptune, similar to Figure 4 in this paper, using values given here. I initially tried using both the Argument of Periapsis, and the Longitude of the Ascending Node separately but neither seems to give the correct results. After a bit more reading I now think I will need to combine both these values to derive the orientation of the major axis of the ellipse, but I can't find the formula anywhere. Am I correct? Could someone show me the correct calculation?


texts: How to approximate the distance at which the thin disk peaks along a given LOS from Earth for a given galactic coordinate per 1 sq deg?

texts:
The density of stars drops off exponentially from the center of the galaxy and can be approximated as follows using a cylindrical coordinate system:
$$ \rho(R, z) = \rho_0 \exp \frac{-R}{h_R} \ \exp  \frac{-|z|}{h_z} $$
screenshot
The radial scale length and height of the thin disk are ~ 3000 pc and ~ 200 pc respectively. At the moment rho_0 will just be taken as 1. We observe at a given l and b of 90 and 15 respectively using the galactic coordinate system (just look at l and b for now, the rest are annotations I added that I will describe shortly). The galactic coordinate system uses spherical coordinates:

We want to know where the density of stars peak along at given LOS (from Earth) per 1 sq degree at a given l and b. There are 41253 sq degrees in the entire sky, so 1/41253 will be the fraction we want to multiply the density equation by. However, we must also consider how the volume element changes by distance along the given LOS, so we need to also multiply rho and 1/41253 by this change in volume. This volume element can be represented by $4 \pi r^2 dr$ (the surface area of the sphere and the small change in the distance along the LOS). We are almost done but need a way to transform between coordinate systems by relating galactic coordinates to distance r and R (as seen in the diagram above). Mathematically, they are related as follows (Eq. 3 in Where Does the Disk Stop and the Halo Begin? Kinematics in a Rotation Field Morrison, Flynn and Freeman 1990 Ast. J. 100 (4) 1191-1222):
$$ r^2 = R_☉^2 + R^2 - 2 R R_☉ \cos b \cos l $$
screenshot
So, for a given r (distance from star or LOS to sun), l and b, we can find R (the distance from the G.C. to the star or LOS). This is in cylindrical coordinates, so we need to use cos(b)R and sin(b)R to find R and z in the density equation above (note these are different r's). The volume element needs to be the LOS from Earth, which corresponds is the R in the diagram and equation for the paper (for the given l,b, and r). Putting this all together, I have the following code. Please let me know if this attempt seems reasonable, or if not, where I went wrong.
import numpy as np
import matplotlib.pyplot as plt

C=8500 #Distance from center of galaxy to sun (R_sun_

#Galactic Coordinates
b=np.deg2rad(15) #above plane
l=np.deg2rad(90)

#Scale heights for the thin disk
h_R=3000
h_z=200

nsqdeg_in_sky=41253 #square deg in whole sky

#R: Distance from star to sun
R=np.arange(0,15000,1)

#distance from star to G.C.
#r: distance from star to G.C. kpc
r=np.sqrt(C**2+R**2-2*R*C*np.cos(b)*np.cos(l))
print(r)

#Spherical Galactic Coordinates to cylindrical coordinates
#z component of R
R_z=np.sin(b)*r
#r component of R
R_r=np.cos(b)*r

#density equation describing thin disk
rho_thin=np.e**(-R_r/h_R)*np.e**(-np.abs(R_z)/h_z)

#desnity equation combining volume element and portion of sky
N_thin = (1./nsqdeg_in_sky)*(4*np.pi)*R*R*rho_thin

plt.title("Number of stars per 1 square degree")
plt.plot(R,N_thin,color='blue',label='thin')
plt.legend(loc='best')
plt.xlabel(r'$distance$ [pc]')
plt.ylabel(r'$N_\ast$ [pc$^{-1}$]')

edit to add plot for given l and b above:


texts: How to calculate a phase shift in the process of deriving Baade-Wesselinke distance?

texts:
The above is the equation used in the Baade-Wesselink method to independently get distances to Cepheid variables (in the process of deriving Period-Luminosity relation).

According to the paper New Baade-Wesselink distances and radii for four metal-rich Galactic Cepheids, δθ in the equation accounts for a phase shift between the radial velocity curve and the angular diameter changes measured via surface-brightness relation. My question is how can I calculate this δθ value given that I have lists of radial velocity values and V-R values with the observed HJD dates (which is possible to be converted in terms of phase).


texts: Uranium accumulating inside a White Dwarf?

texts:
A couple of days ago, there was news that "Some dead stars may harbor enough uranium to set off a thermonuclear bomb", which is basically a teaser for Actinide crystallization and fission reactions in cooling white dwarf stars where the authors conclude:

The first solids that form as a white dwarf (WD) starts to crystallize are greatly enriched in
actinides because of their large charges. We estimate that
these first solids could be so enriched in actinides that
they may support a fission chain reaction. This reaction
could ignite carbon burning and lead to the explosion of
an isolated WD in a thermonuclear supernova.

Going through manuscript, it looks to me that this conclusion is drawn from molecular dynamics simulations  of a cube of up to $10^4$ ions. What exactly is the line of argumentation of these microscopic observations from computer simulations are to macroscopic effects in the star?
I understand, that a main question of the research was, whether a certain scenario is actually possible (inside a white dwarf), or literally

Could uranium, [...], accumulate inside a white dwarf?

Just to clarify: I am not doubting the conclusion or questioning the method per se, I am just curious whether such an approach is usual for studying stellar evolution. Differently put: Is there a standard theory describing the conditions when the conditions are right for a white dwarf to explode?
Related

How are shockwaves in a star that cause supernova created
Stellar life cycle flow chart with mass conditions and time scales


texts: Is T Tauri a compulsory stage for stellar evolution?

texts: Do all stars go from Protostar stage to T Tauri and then to Main Sequence or is T-Tauri stage only for certain stars?  If only certain stars, is that those that are less than 3 solar masses which i see mentioned?  (https://en.wikipedia.org/wiki/T_Tauri_star)


texts: When do radio and tv signals become indistinguishable from background noise of the universe?

texts:
I understand the rate an EM signal broadcast uniformly from the Earth will decrease in its power is governed by the inverse square law. How far from Earth will radio and tv signals become indistinguishable from background noise from varied sources in the galaxy, even CMB.

I know nothing about the type of equipment that would be used to gather and analyze such a signal.  I don't know what the geometry of the observation means.  Assuming the optimal equipment and conditions we currently have and no other line-of-sight sources.


texts:
This should be a straightforward exercise, but I'm struggling. Help is much appreciated

I would like to parameterize a Navarro-Frenk-White profile with one parameter, using the correlation between total mass and the concentration.

As a starting point, I have the density in terms of two parameters: the total dark matter mass inside the virial radius $r_{vir}$, and the concentration parameter $c=r_{vir}/r_{s}$:

$\rho_{DM}(r) = \frac{M_{DM}}{4\pi A(c)}\frac{1}{r(r_{s}+r)^{2}}$

where the function $A(c)$ is given by

$A(c) = \ln{(1+c)} - \frac{c}{1+c} $

Using the correlation between $M_{DM}$ and $c$ (from e.g. Napolitano et al. 2005)

$c(M_{DM}) \approx 9.195 \left(\frac{M_{DM}}{h^{-1}10^{12} M_{\odot}} \right)^{-0.094}$

it should then in principle be possible to parameterize $\rho_{DM}$ as a function of $M_{DM}$ only. This is how they do it in e.g. Williams et al. 2009.

However, I'm not managing to reproduce that. Sure, given the correlation above, $M_{DM}$ determines $A(c)$, but won't $\rho_{DM}$ still depend on $r_{s}$?


texts: How accurate are the new visualisations of V404 Cygni

texts:
A number of pictures and some animations similar to the ones below have had much press recently. Clearly the colours are artificial, but I wondered how accurately the represent the geometry of the system -- the relative sizes of the event horizon and the thin and thick accretion disks, the distance from the companion star, and so on.

Does anyone know.

The images are taken from popular press sites, but seem to originate from ICRAR, where they are described as "artists impressions". I would nevertheless be interested to know how accurate a brief their artists were working from.


texts: How do I get the change in angular diameter value?

texts:
Given that I got the equation relating the angular diameter(θ) and $V$–$K$ band value, which is known as $\logθ_0=(0.262±0.004)(V−K)_0+(0.547±0.006)$ from this paper, how do I get the 'change' in angular diameter value from that equation?

1) Can I just get the max and mix value of the $V$–$K$ value and insert each into the equation and get max θ and min θ and get the difference?

2) Or I need to the whole lists of angular diameter value over each period and insert the lists of value into the equation and plot the graph and then get the difference?

3) Or is there any other method to do this?


texts: Which stars, identifiable with the naked eye, have exoplanets?

texts:
If I look at the night sky, which easily identifiable stars have known exoplanets, and what are the names of those planets?

With the recent discovery of so many exoplanets, it is a nice idea to look at the stars with this new knowledge. They will no longer seem like a bunch of anonymous white dots.


texts: What is the solar system orientation to its direction of travel?

texts:
I saw the picture and seem incorrectly drawn. Its shows the the edge of the solar system leading facing the Heliosheath. It would not be like NASA to post something as incorrect as this. The solar system should at 60 degrees to the direction of travel?
I'm not sure maybe this picture is correct?
https://solarsystem.nasa.gov/news/784/nasas-voyager-2-probe-enters-interstellar-space/


texts: What is the optimal escape trajectory from near a black hole?

texts:
Consider a space ship that is being drawn closer to a black hole.  The crew begins to notice the effects, and discovers that they are nearing the black hole.  They then manage to halt their progression toward the black hole.  What would be the optimal escape trajectory, assuming they have not yet wandered too close?  I am thinking it would likely either be directly away from the black hole (figure 1), or parallel to the black hole (figure 2).  However, it is conceivable that there would be some other angle that would be optimal (e.g. 45 degrees in figure 3).

Figure 1: Go directly away from the black hole

Figure 2: Go "parallel" to the black hole

Figure 3: Go 45 degrees from the black hole

A follow up question is, would this answer be the same for other high gravity objects, such as neutron stars, or even regular stars?

Let's say for simplicity sake that they have already managed to halt their advance toward the black hole and they are now still (not moving) in relation to the black hole. If distance factors in, it would be interesting to see what the answer is at different distances (e.g. fairly close to the event horizon where I realize a tremendous amount of energy would be required, and for far away).


texts:
if the universe transitioned from a meta-stable vacuum to a stable vacuum we assume a universal death scenario. However since it cannot propagate faster than the speed of light, from any particular starting point, the universal expansion has crossed an intrinsic "event horizon" so how can this actually happen in any conceivable time scale?

also - cheeky extra question - could energy rich galaxies dislocate in some way from energy poor regions of spacetime


texts: Problem about finding distance with magnitude given

texts:
A star in Pleiades cluster has apparent magnitude of +12 ($m_{star} = +12$) determine it's distance from the sun? (We know the Sun properties such apparent and absolute magnitudes, distance from earth, luminosity, brightness (energy flux) and others but nothing is given about the other star expect that apparent magnitude).

My problem is that we don't have luminosity of the star. We can say:

$m_{sun} - m_{star} = -2.5 * \log_{10}{\frac{b_{sun}} {b_{star}}} \Rightarrow 10^{(-26.8 - 12)/(-2.5)} \approx 3.31131 * 10^{15} = \frac{b_{sun}} {b_{star}} \approx \frac{1370 Wm^-2}{L_{star} / (4  \pi  d^2)} $

As we can see, we don't have luminosity of the star. Can this question be solved with this information?


texts:
The NPR.org article Hawaii Protesters Block Access Road To Stop Construction Of Massive Telescope briefly summarizes the situation, one paragraph says:

  The TMT, as it is called, has been the object of intense opposition for nearly a decade and the building project has faced numerous delays, protests and lawsuits. But a decision by the state's Supreme Court in October cleared the way for construction to begin after reinstating a building permit that was revoked years earlier. And last week, Gov. David Ige announced several roads would be closed beginning Monday to allow for the movement of large equipment onto the mountain.

and another says

  "Hawaii is a unique place," Meisenzahl said, noting that protesters gathered on the mountain were engaging in a peaceful, nonviolent form of protest called Kapu Aloha. "It's a small island and everybody knows each other so law enforcement and opponents are working together. Despite what is happening today it's a positive environment," he added.

Question: What would be the scientific impact of locating the Thirty Meter Telescope at its alternate site? Is it a loss of resolution, or infrared spectrum, or both, or something else?

I think the Canary Island site would be the alternate to Mauna Kea, since the E-ELT would be located in Chile and putting both (extremely) large telescopes in the same hemisphere is undesirable.


texts:
Here's what I'm curious about. So this hydrogen gas collects and at some point, it eventually becomes a star. What does that process look like? If you were there as a witness to the formation of a star (greatly speeded up, of course), what would you see. What stages would the gas go through? Would it first form a gas giant planet at some point? With perhaps a solid core of something covered with metallic hydrogen and then hydrogen gas over that? Or does it form differently?
I assume at some point a critical pressure/temperature point is reached and fission begins. What does that look like? Does the whole thing just burst into flames or does it take time?
And somewhat relatedly, I'm curious how large stars form. Why wouldn't it first form as a small star? I have always assumed that once the star ignites, it's going to radiate away any of the collecting gas. But maybe that assumption is invalid. Will the gas continue to collect due to the gravity and the nature of the star simply evolves as the gas collects?
I've never really seen a sort of "diary of a star's birth" kind of description.


texts: What happens to oxygen produced on the Sun (or other stars)?

texts:
Through nuclear fusion, the Sun can (or at the very least, someday will) produce atoms of all elements up to and including oxygen.  And in terrestrial chemistry at least, when you combine oxygen, hydrogen, and a small amount of heat you get water.  Or lithium and oxygen and heat will produce lithium oxide.  Those individual components are all readily available in a star like the Sun.

So my questions is, does the oxygen produced by the Sun (or any other star, now, in the future, or in the past) react chemically (for instance, through oxidation or combustion processes) with the other elements present in the star's atmosphere?

Or I suppose more generally, do the elements formed through nuclear fusion in a star chemically interact to produce more complex molecules (and if not, then why not)?


texts: How can the Kuiper belt coexist with the 9th planet?

texts:
There are asteroid belts, and there are collections of bodies in resonant orbits, such as Trojans. Aren't these separate things?

The recent evidence for the 9th planet is an alignment in various parameters of known Kuiper belt objects. Conversely, few or no objects are known in the parts of the belt that would be swept away by the hypothetical planet.

If the Kuiper belt straddles the gravitational neighborhoods of Neptune and this other planet, and its populations reflect their influence, is it really a belt at all? A lopsided belt? Or just a region with a collection of distinct populations, each associated with a dominant body?


texts:
This is for a novel.

A rogue planet, is, by a one-in-a-zillion freak chance, on its way to collide with Earth. It has been drifting in interstellar space since its original formation (billions of years ago), and thus will be about as cold as it can get. It is approaching at a sharp angle to the ecliptic, and at its nearest point it will pass within the orbit of the Moon.

I'm currently envisioning its mass to be roughly equal to Neptune, but the only real constraint is that even if it does not collide with the Earth, its passage will end life on Earth with near certainty.

My question is: How close can such a planet come and still have a reasonable chance of not being detected from Earth by modern astronomy? Assume that the astronomical community is at its usual level of vigilance. I don't want the failure to detect it earlier to be due to some fluke; the oncoming collision is flukey enough.

I assume that this will depend on its composition, whether predominantly rocky or predominantly gaseous. I'd prefer the answer to be based on whichever type can get closer without being detected.


texts: Unfortunately, I'm not any good at figuring these things out myself so hope for help here. If the (true) north pole were over the Keweenaw Peninsula of Northern Michigan, what would the North Star be; a star, similar to Polaris that never sets? Or, what star would be directly overhead? I couldn't even pick out Venus or Mars in the night sky when I was younger and now, for personal reasons, I'm unable to go out and look. Any help would be incredibly appreciated. Thank you


texts: If a hot Jupiter collides with the parent star, does it enhance or reduce the life of the star?

texts: Suppose a hot Jupiter is collided into parent star due to tidal force, does the life of the star becomes longer due to extra hydrogen, or becomes shorter due to extra mass?


texts: Are we moving faster than the Cosmic microwave background radiation?

texts: if the Cosmic microwave background radiation is coming towards us in the speed of light and all matter and energy was singular at the big bang, doesn't that mean we somehow moved faster than the speed of light (otherwise the CMBR would have already passed us not be coming toward us?)


texts: Found green rock 20cm below surface, is this an meteor piece?

texts: I am wondering if this rock on picture below is meteor fragment? If so what composition you think it is? It is magnetic i tried with magnet and it sticks to it very strong, it does have a lot of mass.


texts: What happens when two black holes collide?

texts: There are many hypothesis saying that there will a collision between the milky way and Andromeda galaxy...so what happens when two black holes will collide?? And how will it effect the other objects revolving around them??


texts: Where can I find the previous years' International Astronomy Olympiad question papers?

texts: I have looked at the official site but the link for previous papers isn't working.


texts: Does gravitational radiation change the eccentricity of binary orbits?

texts: Do the eccentricity of a binary orbit change with change in the semimajor axis due to loss of orbital angular momentum by gravitational radiation?


texts: Do moons of rocky oblate planets migrate to low inclination over time? If so, does the time it takes depend on the Moon's size?

texts:
Discussions elsewhere suggest that this happens and that smaller moons would tend to low inclination with respect to the planet's equator faster than larger ones.
For example, Mars' small moons are inclined only by 1° or so while Earth's moon's inclination varies between 23-5° and 23+5° with an 18.6 year period.
But this is not a good example because the origin and history of these moons is very different.
Question: So if we had two Earths around two Suns at 1 AU each, and each had a rocky moon like our own inclined at say 10° with respect to the Earth's oblate equator at the same distances except that one moon was big and one was small:

Would they migrate to low inclination over time?
If so, does the time it takes depend on the Moon's size?
If so, which would tend to migrate to low inclination faster?


texts: Does a planet's rotation speed and distance from its star influence the precession of its axis? And is the Earth's precession "normal"?

texts:
I guess first off, do all/most planets with a tilted axis experience precession? And if so, is it usually in the same direction (clockwise or anticlockwise)?

What I'm really curious about is if a planet is tidally locked to its star, could it still experience precession which would in effect slowly shift the day-night line on the planet?

I'm aware that most planets tidally locked to their stars are under more extreme gravitational forces whether it be a larger star than the sun or an orbital distance much smaller than 1AU. How would these gravitational forces affect the precession?

And lastly, is there any theoretical "limit" to the speed of precession? I know for the Earth it takes the axis ~26,000 years to complete a full circuit. Is it typical that this process takes so much longer than for the Earth to complete a rotation or revolution or could a planet theoretically have a much shorter precession cycle and what kind of effects would such movement have on the planet itself?  And similarly, if the planet's rotation speed is much slower than the Earth due to tidal locking (let's just say ~2-3 months) would the precession eventually stop all together or not until an object stops rotating completely?

If anyone feels like being particularly helpful and doing some math that goes way over my head, perhaps you could give me a rough estimate of the speed of precession (i.e., length of time required for a complete circuit of the axis) for the following situations:

An Earth-sized, moonless planet tidally locked in the goldilocks zone of an average red dwarf;
A Mars-sized, moonless planet with the same conditions.

I've also read that other factors influence the precession such as the gravitational pull of large Jupiter-like planets. Is this minimal or considerable?


texts: Why do transits of Earth across the Sun seen from Mars follow a pattern of occurring after 26, 79 then 100 years?

texts:
According to this answer transits of Earth across the disk of the Sun as seen from Mars:

  The (Arthur C.) Clarke story is quite correct. Earth (and Moon) would transit the sun from the viewpoint of Mars on 8 May 1905, 11 May 1984 and again on 10 Nov 2084.

no surprises there, but they

  ...follow a pattern of occurring after 26, 79 then 100 years.

Why is that exactly? Why this pattern?


texts: Color index B-V of the dimming Betelgeuse

texts:
Using AAVSO light curve generator I have plotted V and B magnitudes of Betelgeuse in the past 150 days to see trend of B-V color index. If dust would be the cause of the recent drop in the star brightness, I would expect redenning (in fact de-blueing) of the starlight, that is increase of B-V value. In fact, B-V changed only slightly and it the opposite direction. In case of interstellar reddening, I thinks that change of V magnitude is roughly 3 times greater than the change of B-V color index, which is not the case here.

Is the change of B-V color index of Betelgeuse compatible with expected properties of circumstellar dust?


texts: I was reading Gene Roddenberry's guide for TNG writers and he mentions that that the time of ST:TOS, only 4% of the stars in our galaxy have been charted; this goes up to 19% by the time of TNG. This got me to wondering: what percentage of stars in the Milky Way galaxy have actually been catalogued?


texts: How to invert fits headers order (primary/secondary)?

texts:
I have an image from Hubble telescope with two headers and this creates a problem because my program expects the NAXIS, NAXIS1, NAXIS2 in the first header, but in this image case, this info is in the second header.
So what I need is to invert the order of headers. i.e, SCI header should come first.
When I run:
from astropy.io import fits
image = 'image.fits'
fits.info(image)

I get
No.    Name      Ver    Type      Cards   Dimensions   Format
  0  PRIMARY       1 PrimaryHDU     722   ()
  1  SCI           1 ImageHDU       110   (1900, 1900)   float32

I already tried editing the first header, to add the needed info, but for some reason, it won't accept the changes.


texts: What are the advantages and disadvantages of a 2-inch eyepiece versus a 1.25-inch eyepiece?

texts: I'm considering picking up some new eyepieces for my telescope and am interested in comparing 2-inch and 1.25-inch eyepieces. Are there any disadvantages to 2-inch eyepieces? Are 2-inch eyepieces only advantageous with longer eyepieces?


texts: Does the CMB pattern evolve in a human lifetime?

texts:
I was thinking, as CMB is a snapshot of a slice of the Universe during recombination, how much the cmb is changing with time ?
I mean as we progress in time we look a CMB a bit more faraway each time, could this effect be seen and could we watch the pattern of CMB anisotropies changing with time (on a "human" timescale I mean) ?


texts: Will the shape of the constellations change if we observe from another planet, say Mars?

texts:
Constellations are dominant features of night sky. But these are basically group of stars that forms particular shapes when viewed from Earth.

How much the shape of a constellation will change if we observe them from another planet, say, Mars?

If no significant change be visible, then how much distance we need to travel to observe a change?


texts: What is the escape velocity of our Galaxy relative to?

texts:
I've been thinking that everything has a relative escape velocity, if it has Mass, seemingly. You can escape orbit around the earth by accelerating enough. You can escape the orbit of the sun by accelerating enough. Heck, this is kind of a stretch, but even electrons can escape an atom relative to how much electric charge the nucleus holds. What would it be like to escape the orbital velocity of our Galaxy-- and what exactly would it be relative to-- a black hole at the center of the Galaxy?

The quote was:

  Solar system's orbital velocity is estimated at roughly 220 km/s, and galactic escape velocity for our vicinity at about 537 km/s. So in the direction of Solar system's velocity vector, velocity required to escape Milky Way is ~ 317 km/s. And much more, if this Solar system's own orbital momentum cannot be used to full extent and a launch in other directions is required. This is of course assuming you can launch on a trajectory that avoids getting too close to gravitational influence of other solar systems.

How in the world do you calculate the escape velocity of other galaxies let alone our own-- do we know what the solar systems in the Milky Way orbit and how they orbit it?


texts: Angular diameter of the Sun's reflection from the ocean, seen from Sun-Earth L1?

texts:
I'm trying to understand how smooth the reflecting ocean surface would need to be to produce such a small bright spot as seen from the DSCOVR satellite at Sun-Earth L1. It appears to be only about 8E-05 rad, or about 0.3 arcminutes.

My question is primarily about the geometrical optics involved in the reflection of light from the Sun off of the Earth. Interpretation of the apparent size and comparing to the image is secondary.

At a distance of 151.3 million km from Earth, the 1.391 million km diameter Sun has an angular width of 0.009196 radians. The DSCOVR satellite is 1.586 million km from the Earth looking at the Sun's reflection from somewhere around the Sun-Earth L1 point. It's close to the Earth-Sun axis (the Sun-Earth-DSCOVR angle is about 7.6 degrees) so let's simplify and assume it sits on-axis.

The Earth's radius is 6378 km; as a convex mirror its focal length is $f=-r/2$ or -3189 km.

Question: What would be the angular size of the Sun seen from DSCOVR reflecting in a spherical mirror model for the Earth in this configuration?

What I'm looking at (for context):

This sequence of images taken by the EPIC camera on the DSCOVR satellite near the Sun-Earth L1 point is notable for showing the progress of the Moon's shadow across the Earth, but that's not the topic of my question.

The first image shows the back-reflection of the Sun from the Earth. It is likely to be from a particularly calm spot on the Ocean, but that's not for sure, there are some images that show reflections from horizontally oriented ice crystals as well. See Are there measurements or calculations that suggest atmospheric ice plates would be horizontal to within 0.1 degrees? and also How could the recently explained “glints” seen by DSCOVR appear so compact considering the finite size of the sun? for some interesting images and more on that.

But that doesn't matter for the purposes of my question.

Width of Earth 6378 km x 2 / 1586000 km = 8.0428E-03 rad corresponds to ~1500 pixels.

Bright spot is about 15 pixels wide, or about 8E-05 rad.

Could such a small angular width be a geometrical reflection of the Sun from a very smooth ocean?

Raw image at NASA seen here.


texts: SpaceX's Starlink Megaconstellation and its Effect on Earth-Based Astronomy

texts:
SpaceX's Starlink project is already greatly impeding scientists ability to make good Earth-based observations. With the planned launch of 12,000 satellites total, the future is uncertain.
Is there any hope that "DarkSat" or the new "VisorSat" will make any noticeable difference in the brightness of the sat constellation? DarkSat was observed to decrease the brightness of the satellites by 50%, but they are still highly visible to telescopes. Has SpaceX maximized its ingenuity?
Some have proposed "lowering" the orbit of the satellites ( I'm assuming to decrease the angle the Sun has to catch them as they orbit). Is there any LEO orbit that could actually fix the issue of the Starlinks ruining precious observation time?
Is there any future in which SpaceX AND companies who hope to launch their own megaconstellations can eliminate or greatly mitigate the impact the sats have on Earth-based astronomy?


texts: Is there available space images taken by the south pole Gregorian telescope?

texts:
I searched the internet for images taken by the South Pole Telescope but without successes in finding it.

Edit:Some images


texts: How do we know the universe's expansion is speeding up?

texts:
Please forgive my ignorance, I am not an astronomer, just an interested layman.

The further away a galaxy is from us, the faster it is moving away from us.  But the galaxies we see exist in the distant past because it takes a long time for their light to reach us. The closer galaxies are, the more recent their light is, and the slower they move away from us. Doesn’t this seem to indicate that the expansion of the universe is slowing down, because the more recent light from galaxies, appears to be moving away slower? Does this make sense?

Quote from Wikipedia:

  Hubble observed that the distances to faraway galaxies were strongly correlated with their redshifts. This was interpreted to mean that all distant galaxies and clusters are receding away from our vantage point with an apparent velocity proportional to their distance: that is, the farther they are, the faster they move away from us, regardless of direction.


texts: Pinhole Projector: Can I use a specific solar optical eye piece to focus the Transit of Mercury?

texts:
The title pretty much explains this one. Can I use a specific solar optical eye piece, or other optical device, in conjunction with the pinhole projector method to focus the Transit of Mercury?

Would anyone have any specific recommendations for specifications, etc if it is possible to achieve this...


texts:
Today I try to watch Venus through a mirror telescope. I am no professional. No matter what lens I take, I see this image: What am I doing wrong?

Edit: What happens when I refocus:Sorry for the bad quality, I didn't manage to make better images. :-(


texts: What is germane doing in the atmosphere of Jupiter?

texts:
Germane (GeH4) has been detected in the atmosphere Jupiter.

Its formation from the elements seems to be thermodynamically unfavorable because its enthalpy of formation is positive and (I think) it has lower entropy than the free elements.

So what the heck is it doing there, considering free germanium would sink and free hydrogen would float?


texts: What would the CMB look like somewhere else in the universe?

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An answer to a question on Worldbuilding.SE opined that if you were suddenly transported an unknown distance and direction away by magic you could use the Cosmic Microwave Background to determine your position relative to the Earth.

This lead me to wonder, what does the CMB look like at different positions in the universe? Can changes in how the CMB looks be used to determine your position, direction, or velocity relative to Earth?